Tapered light guide plate for surface light source device and method of making by injection molding via supplementary cavity

ABSTRACT

There is provided a very thin light guide plate for a surface light source device and a method of manufacturing it without the use of high-precision molding machine. A surface light source device using this light guide plate generates outgoing light having high uniformity. According to the manufacturing method, a light guide plate free from a weld line or warp can be manufactured with good transferring characteristics. The light guide plate is thick on an incident surface  1  side and thin on a lower surface  4  side. At the central portion of an incident surface  1  in a longitudinal direction, a projecting portion obtained by cutting an overhang portion  7  at a position a distance D apart from the incident surface  1  is formed. The cut surface of the projecting portion is not made specular and is kept rough. In molding of the light guide plate, a molten material is supplied from the position of a gate mark  9.  After molding, the overhang portion  7  is cut off by an ordinary cutter, thereby obtaining a light guide plate.

Multiple reissue applications have been filed for U.S. Pat. No.5,967,637. This application is a divisional reissue of application Ser.No. 09/977,241 filed Oct. 16, 2001, issued as Re 40,146, which is areissue application of application Ser. No. 08/520,648 filed Aug. 29,1995, now U.S. Pat. No. 5,967,637 issued Oct. 19, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light guide plate for surface lightsource device and a method of manufacturing it and, more particularly, alight guide plate for a surface light source device used as a backlightfor a liquid crystal display for OA equipment, a television set, ameasuring instrument, a watch, or the like, and a method of the lightguide plate by injection molding.

2. Description of Related Art

A surface light source device is well known. A small surface lightsource device is used as a backlight for a liquid crystal displaywristwatch, and a large surface light source device is used as abacklight for an advertising display panel or an illumination device fora show window. An LED is used as the light source for the small device,and a fluorescent tube is used as the light source for the large device.As a light guide plate, an acrylic plate cut so as to have a properdimension is used.

Since a surface light source device is required to have a smallthickness and to cause a predetermined plane to emit uniform light, alight source is generally arranged at a lateral position of the lightguide plate. For this reason, the light guide plate has been subject tovarious processings which include shaping to a special shape, surfaceroughening with sandpaper, special tool or apparatus and polishing ofincident surface to making it specular.

In recent years, with epochmaking progress of a liquid crystal displaytechnique and development of OA equipment, electronic-communicationequipment, or the like, demand for a surface light source device used ina liquid crystal display device having a size of about 10 inches steeplyincreases. As a light source arranged at a lateral position of a lightguide plate, a long-life extremely slender fluorescent tube having adiameter of 4 mm or less is developed. Light guide plates have come tobe manufactured by injection molding which has a small number of stepsand provides mass-production of light guide plates with stable quality.

A light guide plate used in a surface light source device is required tobe variously improved to make it possible that a plane having a designedarea emits source light guided in a latitudinal direction as uniformplane light. Various proposals associated with the improvements havebeen made. Almost all the various proposals use one of the followingtechniques: a technique in which a surface opposing the emissionsurface, i.e., a reflecting surface, is processed (formation of anuneven surface, coating, or printing) by some means to modify itsreflectance distribution; a technique in which a reflecting surface isarranged not parallel to an emission surface, but formed by variousplanes and curves; and a technique obtained by combining both the abovetechniques to each other.

In these techniques, a light guide plate having a reflecting surface notparallel to the emission surface is thick at the incident surface forsource light or near the incident surface and thin at positions distantfrom the incident surface. Such light guide plates are disclosed inJapanese Patent Laid-open No. 3-59526, Japanese Utility Model Laid-openNo. 3-104906, Japanese Utility Model Laid-open No. 5-75738, or JapaneseUtility Model Laid-open No. 5-75739, and in FIG. 7, FIG. 8. FIGS. 7 and8 are side views showing light guide plates. Referring to FIGS. 7 and 8,each upper surface is the incident surface, and each left surface is theemission surface. The present invention relates to a light guide platehaving a shape as shown in FIG. 7 or 8 and to an improvement of a methodof manufacturing the light guide plate.

A conventional method of molding a light guide plate is described herewith reference to a light guide plate having a typical shape shown inFIGS. 9 and 10. FIG. 9 is a plan view showing the light guide plate whenviewed from the emission surface side, and FIG. 10 is a right-side viewof FIG. 9. Therefore, referring to FIGS. 9 and 10, an upper surface isthe incident surface 1 which is perpendicular to the emission surface 2in FIG. 10. Since a fluorescent tube is arranged in the longitudinaldirection of the incident surface at a position outside and near theincident surface, the thickness of the light guide plate is defined inconsideration of the diameter of the fluorescent tube.

A reflecting surface 3 is obliquely formed with respect to the emissionsurface 2 so that the reflecting surface 3 can directly reflect incidentlight from the incident surface 1. Thus, the thickness of the guideplate gradually decreases downward in FIGS. 9 and 10.

When the light guide plate is to be formed by injection molding, aspecific position on a specific surface for a gate (opening throughwhich molten material is injected into a mold cavity) of the mold mustbe determined. Since the light guide plate is used in a surface lightsource device, the gate position is determined in consideration of theutilization efficiency of light, uniform emission from a large emissionsurface, profitability and so on. Judging from this point of view, it isconventionally assumed that the incident surface should be specular toefficiently receive source light. In addition, all the surface should bespecular to obtain uniform outgoing light.

According to conventional understanding, a gate arranged on the incidentsurface provides an increase in cost because the incident surface mustbe made specular by high-accuracy polishing or buff finish serving aspost-processing. Therefore, such arrangement of gate on the incidentsurface has not been employed.

The emission and reflecting surfaces generally must have effective areasas large as possible. In addition, in order to obtain uniform outgoinglight, an uneven surface having a variety of shapes such as a net-likepattern is often formed on the reflecting surface, otherwise coating orprinting is often applied to the reflecting surface. Therefore, it hasbeen avoided to arrange a gate on these surfaces.

Referring to FIGS. 9 and 10, a gate may be arranged on the lower surface4. However, the light guide plate has a very small thickness at lowersurface 4. For example, an ultra-thin light guide plate for officeAutomation (“OA”) equipment having a size of about 10 inches used, thethickness at the lower surface is about 1 to 2 mm, and a gate can hardlybe arranged on the lower surface. Even if a gate is arranged on thelower surface, a sufficient injection pressure cannot be obtained by ageneral molding machine, an acrylic molten material cannot be preferablyinjected into the cavity. Thus, transferring characteristics areconsiderably degraded.

In order to increase the injection pressure, or increase the temperatureof the mold to make the flow of the material easy, a highly expensivemolding machine of high-accuracy control is required. For this reason,the cost of the light guide plate inevitably increases. Furthermore, thenumber of gates may be increased. In this case, however, the cost of themold increases and a so-called weld line is inevitably formed, leadingto a fatal problem for uniform outgoing light. Therefore, it is notpractical to arrange a gate on the lower surface.

For these reasons, a gate is conventionally arranged at a position onone of side surfaces 5 and 6 in FIG. 9, in particular, in the thickportion near the incident surface. Similarly, the above arrangement isemployed in molding for a two-light light guide plate in which theinclination direction of the reflecting surface 3 with respect to theemission surface 2 is reversed at the central portion of the reflectingsurface 3, the lower surface 4 has a thickness almost equal to that ofthe incident surface 1 so that another fluorescent tube is arrangedoutside and near the lower surface 4, as shown in FIG. 11.

However, according to a conventional molding method, as shown in FIG. 9,if a gate G is arranged on the side surface 5, the material quicklyflows in a flow path A, but slowly flows in a flow path B at injection.Thus, the material does not uniformly flow from the incident surface 1to the lower surface 4 and the flow varies depending on the position. Inaddition, the flow on the side surface 5 is considerably different fromthe flow on the side surface 6. For this reason, the pressure differenceand temperature difference between regions fall into disorder.

As a result, in particular, when an uneven surface having a variety ofshapes is formed on the reflecting surface, the shape of the unevensurface cannot be desirably transferred. In addition, problems such asformation of an weld line and warp after molding arise easily. In theultra-thin light guide plate as shown in FIGS. 9 and 10, since amaterial cannot completely filled in a cavity by molding machine and anordinary molding method, a troublesome examination must be performed forconditions set for molding processing at every slight change in shape ofthe light guide plate.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light guide platefor a surface light source device, which can be manufactured easily byusing a ordinary molding machine on the basis of ordinary conditions anda method of manufacturing it.

In order to achieve the above object, in a light guide plate which ismanufactured by injection molding and has an incident surface in thelongitudinal direction of a slender beam, a large thickness near theincident surface, a thickness decreasing as distant therefrom, and anemission surface for emitting incident light from the incident surfacein a direction perpendicular to the longitudinal direction of theincident surface, the incident surface is partially or wholly formed bya cut surface.

In the light guide plate for the surface light source device accordingto the present invention, preferably, the cut surface is nearlysymmetrically formed in the longitudinal direction with respect to analmost central portion of the incident portion in its longitudinaldirection, and the cut portion is formed on a projecting portionparallel to the other portion of the incident surface. The dimension ofthe projection is set to be about 1 mm or less, preferably, 0.5 mm orless, and the surface roughness of the projection is set to be 50 μm orless, preferably, 20 μm or less in a unit of 10-point average roughness(Rz).

In addition, according to the method of manufacturing the light guideplate for the surface light source device according to the presentinvention, a gate is arranged at a designed position which is the nearlycentral position of the incident surface and overhangs or protrudes fromthe position parallel to the emission surface in the molding mold, and acavity in which a molten material injected from the gate nearlysymmetrically flows in the longitudinal direction of the incidentsurface is supplementarily formed, wherein the molten material is moldedby said mold, and then the overhang portion (also referred to asprotruding portion) molded by the supplementary cavity is cut to form acut surface.

In the above manufacturing method, the gate is preferably formed asfacing an extension of the emission surface or a surface parallel to theemission surface.

A light guide plate for the surface light source device according to thepresent invention is preferably manufactured by the following method. Ina mold to be used, at a nearly central position of the incident surfaceof the light guide plate to be manufactured and distant from theincident surface by a designed dimension, the gate is arranged towardthe same plane as that of the emission surface of the light guide plateor a plane parallel thereto and a supplementary cavity portion is formedsuch that a material injected from the gate symmetrically flows in thelongitudinal direction of the incident surface.

Therefore, the material injected from the gate advances and spreads inthe supplementary cavity as curving at 90° and the material flows intothe cavity portion forming a thick portion near the incident surface ata nearly uniform pressure from an area larger than that of the gate.Thereafter, the material flows parallel from the thick portion to adistal thin portion to fill the cavity. For this reason, approximatelyeven and regular flow and nearly uniform pressure can be obtained in theregions of the cavity. Thus incomplete filling and the formation of aweld line can be prevented, with the result that the shape of the roughreflection surface can be accurately transferred to the product.

After molding, the unnecessary portion molded by the supplementarycavity is cut off to manufacture a light guide plate. The cut surface isused without changing its state and need not be subjected to specularfinish.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing the semi-finish state of a light guideplate according to an embodiment of the present invention;

FIG. 2 is a right-side view showing the semi-finish state of the lightguide plate shown in FIG. 1;

FIG. 3 is a right-side view showing the finish state of the light guideplate shown in FIG. 1;

FIG. 4 is a graph showing the relationship between the surface roughnessof the cut surface of the light guide plate and the luminance of anemission surface;

FIG. 5 is a right-side view showing the finish state of the light guideplate in FIG. 3 in which an overhang portion 7 in FIG. 1 is completelycut off;

FIG. 6 is a right-side view similar to FIG. 3 showing the finish stateof the light guide plate in which the shape of a projecting portion 8 ismodified;

FIG. 7 is a side view showing a light guide plate having a shapedifferent from that of the light guide plate according to the embodimentshown in FIG. 1 and so on;

FIG. 8 is a side view showing a light guide plate having a shape furtherdifferent from the light guide plate according to the embodiment shownin FIG. 1 and the like and the light guide plate shown in FIG. 7;

FIG. 9 is a plan view for explaining a prior art with respect to a casewherein an ultra-thin light guide plate similar to that of theembodiment shown in FIG. 1 and the like is molded;

FIG. 10 is a right-side view for explaining a prior art with respect toa case wherein an ultra-thin light guide plate similar to that of theembodiment shown in FIG. 1 and the like is molded, similarly as in FIG.9; and

FIG. 11 is a side view showing an ultra-thin light guide plate havingthe shape illustrated in FIG. 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will be described belowwith reference to FIGS. 1 to 4. FIG. 1 is a plan view showing a lightguide plate in a state in which the material of the runner portion isremoved after molding; FIG. 2 is a right-side view of FIG. 1; and FIG. 3is a right-side view showing the light guide plate in a finish state inwhich the unnecessary portion is cut off. FIG. 4 is a graph showing therelationship between the surface roughness of the cut surface andluminance. In all of the drawings, the same reference numerals denotethe same parts in.

The light guide plate for the surface light source device according tothis embodiment is similar to the ultra-thin light guide plate asdescribed in FIGS. 9 and 10. This light guide plate has the followingshape. That is, referring to FIG. 1, an overhang portion 7 is cut at aposition distant from an incident surface 1 by a designed dimension D,and this cut surface is also used as an incident surface for sourcelight. As is apparent from FIG. 3, the remaining portion of the overhangportion 7 after the cutting is formed as a projecting portion 8 having athickness equal to that of the thick portion, and the cut surface isformed as a surface parallel to the other portion of the incidentsurface.

Referring to FIGS. 2 and 3, although a reflecting surface 3 is shown asa flat surface, the reflecting surface 3 really has a fine uneven shape.The reflecting surface 3 is designed to reflect source light incidentfrom the incident surface in many directions and to cause the emissionsurface 2 to finally emit uniform light. Although various improvedshapes are proposed as the uneven shape, descriptions thereof areomitted here. Referring to FIG. 1, a gate mark 9 is formed in theoverhang portion 7. As is apparent from this, a gate G in the mold isarranged as to face one surface of the overhang portion 7, i.e., thesame plane as that of the emission surface 2.

Therefore, the material supplied from a runner R is injected from thegate G into a cavity and then the course of the material is bent atabout 90°.

As described above, an ultra-thin light guide plate is used in thisembodiment and the size of the light guide plate will be describedbelow. Referring to FIG. 1, the lengths of the incident surface 1 andthe lower surface 4 are 180 mm respectively, and the lengths of sidesurfaces 5 and 6 are 143 mm respectively. Referring to FIG. 3, the thickportion has a thickness of 3.5 mm, and the thin portion has a thicknessof 1.5 mm. The projecting dimension D of the projecting portion 8 is 0.3mm. A distance between the center of the gate mark 9 and the incidentsurface 1 is 18 mm.

A method of manufacturing the light guide plate by injection moldingwill be described below. The manufacturing method is to be describedwith reference to a mold structure, as a matter of course. However, acomplex drawing is omitted here and the semi-finish state of the lightguide plate shown in FIGS. 1 and 2 will be compared to a molding cavityfor the sake of descriptive convenience. Therefore, the cavity forforming the overhang portion 7 is called a supplementary cavity becausethe overhang portion 7 is cut off after molding as described above.

In molding, referring to FIG. 2, the acrylic-resin-based molten materialinjected from the gate G through the runner R advances in thesupplementary cavity as curving 90°. Since the supplementary cavity hasa width which increases along the direction of the advance, the materialflows into the cavity near the incident surface from an area larger thanthat of the gate G at a uniform pressure. Thereafter, although themolten material laterally spreads in the form of a fan, the moltenmaterial, as a whole, flows from the thick portion on the incidentsurface side to the thin portion on the lower side portion. Although thedirection of the flow is not exactly parallel, the molten material flowsat the degree of parallelization considerably higher than that of theprior art described in FIG. 9, at a uniform pressure.

In this manner, according to this embodiment, although the ultra-thinlight guide plate is made, the material regularly flows in the regionsin the cavity by an ordinary molding machine under ordinary pressurecontrol and temperature control to fill the cavity and corners thereofcompletely. In addition, the uneven shape formed on the reflectingsurface can be well transferred, no weld line is formed and warping isavoided in spite of the ultra-thin light guide plate.

In this embodiment, attention must be paid to that the flowing portionfrom the supplementary cavity to the thick portion has a length which isabout 1/7 the length of the incident surface 1. In order to obtain afurther well material flow, the ratio of the length of the flowingportion to the length of the incident surface 1 may be increased, andthe supplementary cavity may be formed as indicated exemplary by atwo-dot chain line in FIG. 1. In this case, as in another case describedlater, the uniformity of light emitted from the emission surface has noabnormality.

After the semi-finish structure is molded, the overhang portion 7 iscut. This cutting may be performed by using various cutters including adiamond cutter. In this embodiment, the cutting was performed by aconventional cutter having a rotary blade for cutting an acrylicmaterial. Referring to FIG. 5, it is ideal that this cut surface is onthe same plane as that of the incident surface 1. However, inconsideration of mass production, the cutting is performed at a positiondistant from the incident surface 1 by 0.3 mm.

In the light guide plate manufactured as described above, the incidentsurface portion, other than the cut surface, is formed as a specularsurface. Therefore, it must be examined whether the presence of theprojecting portion 8 and the roughness of the cut surface prevent theemission surface 2 from emitting light having a uniform distribution.Thus, the examination result is described below.

A light guide plate having the above size and a cut surface having asurface roughness of 10 μm in unit of 10-point average roughness (Rz)and a light guide plate having the above size and a cut surface having asurface roughness of 60 μm in the unit were prepared. The luminance(cd/m²) of light emitted from emission surfaces were measured, and theluminance are compared with a measurement value for the specular surfaceobtained by polishing the cut surface. The results are shown in thefollowing table. In this measurement, a cold cathode tube having adiameter of 3 mm and a tube surface luminance of 13,700 cd/m² wasarranged 1 mm distant from the incident surface 1 in the longitudinaldirection of the incident surface 1. A lamp holder having an innersurface of a silver reflecting side opposite to the incident surfaceopposing the surface with respect to the cold cathode tube. In addition,a sheet having a front surface on which a silver reflecting surface wasformed was arranged along the rear side of the reflecting surface 3 ofthe light guide plate such that the silver reflecting surface was almostparallel to the reflecting surface 3 of the light guide plate. Threepositions located near the central portion of the incident surface anddistant from the incident surface toward the lower surface 4 were set,respectively.

TABLE Surface Roughness Measurement Position (Rz) 10 mm 71 mm 132 mm 10μm 691 972 681 60 μm 748 633 593 Specular 696 699 672 Surface

According to the above measurement results, it was clarified thatluminance uniformity was considerably degraded at a surface roughness of60 μm, and that the luminance uniformity at a surface roughness of 10 μmwas almost equal to the luminance uniformity obtained by performingspecular finish. It was also clarified that the measurement resultobtained by performing the specular finish was not influenced by thepresence of the projecting portion.

Since the above results were obtained, cut surfaces having four surfaceroughnesses of 20 μm, 30 μm, 40 μm, and 50 μm were prepared to measureluminance at the position of 10 mm where abnormal issue occurred at thesurface roughness of 60 μm. FIG. 4 is a graph on which measurementresults obtained by a total of 7 cut surfaces are plotted.

As a result, it was clarified that the surface roughness of the cutsurface was set to be 50 μm without any problem in the light guide platehaving the size of this embodiment. When measurement was performed suchthat the projecting dimension (D) of the projecting portion 8 in thelight guide plate having a cut surface having a surface roughness of 50μm was set to be 1 mm, the almost same result as described above couldbe obtained. For this reason, it was estimated that the projectingdimension could be further increased without any problem. However, whenthe projecting dimension was excessively large, the fluorescent tubeshould be away from the incident surface by a distance corresponding tothe excessive dimension, thereby increasing a surface light sourcedevice in size. Therefore, in consideration of manufacturing efficiency,the projecting dimension is preferably set to be about 1 mm or less.

In the above embodiment, as shown in FIG. 2, it should be noted that thegate G faces the plane extending from the emission surface 2. One reasonwhy this arrangement is employed is that the following point isconsidered. That is, as is apparent from the size of the gate mark 9shown in FIG. 1, the size of the gate G of the ultra-thin light guideplate can be determined regardless of the thickness of the light guideplate. Another reason is that, when a material widely flows from thesupplementary cavity to the thick portion, the pressure acting on thematerial can be advantageously averaged to some extent. However, thepresent invention is not limited to this arrangement, and the gate G maybe or arranged downward in FIG. 2.

In addition, the thickness of the projecting portion 8 (i.e., thethickness of the overhang portion 7) need not be equal to the thicknessof the thick portion as shown in FIG. 3. As shown in FIG. 6, thethickness may be decreased and the cut surface may be formed on aportion of the incident surface with respect to the latitudinaldirection. In addition, the whole incident surface may be formed as acut surface. In this case, the luminance slightly decreases as a whole,and material loss caused by cutting off the overhang portion increases.For this reason, in the light guide plate having the size described inthe above embodiment, the thickness of the projecting portion 8 ispreferably set to be 25 to 35 mm with respect to the longitudinaldimension of the incident surface, i.e., 180 mm.

The position of the cut surface or the gate position is ideally locatedjust at the central portion of the light guide plate in the lateraldirection as shown in FIG. 1. However, if the position is laterallymoved to some extent, the uniformity of outgoing light is rarelyinfluenced.

In addition, in the present invention, as shown in FIG. 1, overhangportions 5a and 6a each having a proper thickness may be arranged on theside surfaces 5 and 6, respectively. The overhang portions 5a and 6a aregenerally used to position the light guide plate when the light guideplate is incorporated in the surface light source device. In the lightguide plate manufactured by the conventional method shown in FIG. 9,there is trouble in filling the material into the thin portion. For thisreason, when the overhang portions 5a and 6a are to be arranged near thethin portion, the shape of the thin portion is not desirablytransferred, with the result that the distribution of outgoing light inthe thin portion cannot be made uniform.

However, according to the present invention, the above problem can besolved. In this case, the reason why the overhang portions 5a and 6a aredesirably arranged near the thin portion of the light guide plate isdescribed below with reference to experiment data.

The following table shows results obtained by measuring luminancedistribution on the emission surfaces of an 8-inch (133.6 mm×175.5 mm)light guide plate having thicknesses of 3 mm on the incident surface and1 mm on the lower surface and a 9-inch (160 mm×220 mm) light guide platehaving thicknesses of 3 mm on the incident surface and 1 mm on the lowersurface.

A case wherein the overhang portions 5a and 6a are arranged at the upperend positions of the side surfaces 5 and 6 is represented by A; anothercase wherein the overhang portions 5a and 6a are arranged at positionswhich is ⅓ the distance between the incident surface 1 and the lowersurface 4 apart from the incident surface 1 is represented by B, and theother case wherein the overhang portions 5a and 6a are arranged at thelower end positions is represented by C. In addition, measurementpositions are defined as follows.

On the emission surface 2, [1], [2] and [3] are laterally aligned at aposition 10 mm distant from the incident surface 1, [4], [5] and [6] arelaterally aligned at an intermediate position between the incidentsurface 1 and the lower surface 4, and [7], [8] and [9] are laterallyaligned at a position 10 mm distant from the lower surface 4. At thesame time, the following conditions are satisfied. That is, [1], [4] and[7] are vertically aligned at a position 10 mm distant from the sidesurface 5, [2], [5] and [8] are vertically aligned at an intermediateposition between the side surfaces 5 and 6, and [3], [6] and [9] arevertically aligned at a position 10 mm distant from the side surface 6.

In this case, the surface roughness of the cut surface of the overhangportion 7 is 10 μm in a unit of 10-point average roughness (Rz). Thetube surface luminance of the cold cathode tube is 19,500 cd/m² when the8-inch light guide plate is used, and is 23,900 cd/m² when the 9-inchlight guide plate is used. Other experiment conditions are the same asthose described in the above example.

TABLE In Case of 8 Inches (cd/m²) Position of Over- hang MeasurementPosition Portion [1] [2] [3] [4] [5] [6] [7] [8] [9] A 964 1078 980 10601100 1065 1090 1080 1085 B 995 1678 1020 1055 1100 1058 1060 1080 1065 C1040 1078 1050 1050 1100 1055 1038 1080 1035

TABLE In Case of 9 Inches (cd/m²) Position of Over- hang MeasurementPosition Portion [1] [2] [3] [4] [5] [6] [7] [8] [9] A 1030 1127 10351150 1150 1148 1165 1130 1170 B 1060 1127 1075 1142 1150 1145 1100 11301110 C 1110 1127 1116 1135 1150 1138 1115 1130 1120

As is apparent from the above two tables, when the overhang portions 5aand 6a are closer to the incident surface 1, the luminance at positionsnear both the ends ([1] and [3]) near the incident surface 1 muchdecrease. Therefore, in consideration of the uniformity of the luminancedistribution on the emission surface 2, the overhang portions 5a and 6aare preferably arranged at positions distant from the incident surface 1and located on the side of lower surface 4 with respect to at least theintermediate position between the incident surface 1 and the lowersurface 4, i.e., positions near the thin portion.

According to the present invention, assume that a mold having astructure in which a molten material injected from the gate flows fromthe incident surface 1 to the lower surface 4 is used. In this case,when the overhang portions 5a and 6a are arranged on the side of lowersurface 4 with respect to the intermediate position between the incidentsurface 1 and the lower surface 4, the overhang portions 5a and 6ahaving good shape transferring characteristic and a high shape accuracycan be easily formed. When the shape accuracy of the overhang portions5a and 6a is high as described above, an accuracy in positioning thelight guide plate can be increased.

The relationships between positions (A, B, C) of the overhang portions5a and 6a with respect to the incident surface 1 and the decrease inluminance near both the ends near the incident surface 1 are the same asthose obtained for a light guide plate having a uniform thickness fromthe incident surface 1 to the lower surface 4. For this reason, also inthis case, the mold preferably has the structure in which the moltenmaterial injected from the plate flows from the incident surface 1 tothe lower surface 4 and the overhang portions 5a, 6a are desirablyarranged on the side of lower surface 4 with respect to the intermediateposition between the incident surface 1 and the lower surface 4.Although the overhang portions 5a and 6a are arranged on the sidesurfaces 5 and 6 in FIG. 1, the overhang portions 5a and 6a may bearranged on the emission surface 2, the reflecting surface 3, or thelower surface 4 depending on the situation.

In addition, the shape of a light guide plate to which the presentinvention can be applied is not limited to the shape of the light guideplate described in this embodiment as a matter of course. Morespecifically, the present invention can be applied to the light guideplates having the shapes described in the above-mentioned knownpublications and the light guide plates having the shapes shown in FIGS.7, 8 or 11. When the light guide plate having the shape shown in FIG. 11is used, it should be noted that two upper and lower incident surfacesare formed.

In this case, when a mold structure is designed such that the moltenmaterial flows from one incident surface to the other incident surface,the material flows almost in the same manner as the embodiment shown inFIG. 1 while a gradual changing from a fan shape flow pattern with someextent into a parallel flow pattern is caused until it reaches thecentral position in the flow direction, i.e. the thin portion. Passingthrough the central position, a fan-like flow pattern tends to appearagain because of the increasing thickness.

However, in general, this tendency is very weak, with the result thatfilling is completely realized substantially in the same manner as theembodiment shown in FIG. 1, with the parallel flow pattern kept.

It will be understood easily from the above detailed description thatthe present invention provides light guide plates for a surface lightsource device, including ones of ultra-thin types, which are not subjectto incomplete filling in the cavity of the used mold, incompletetransferring of unevenness of various shapes and generation of weld lineor warping, and further which enable uniform light emission.

1. A light guide plate for a surface light source device, manufacturedby injection molding, comprising: an incident surface in a longitudinaldirection of a slender light source; and an emission surface foremitting incident light received at said incident surface, in adirection perpendicular to the longitudinal direction of said incidentsurface, said light guide plate being large in thickness at a positionnear said incident surface and decreasing in thickness with increasingdistance from the incident surface, wherein said incident surface ispartially or entirely formed as a cut surface which remains aftercutting a protruding portion from the light guide plate, the protrudingportion having a supplemental cavity around an injection molding gate.2. A plate according to claim 1, wherein said cut surface issymmetrically formed with respect to an nearly central portion of saidincident surface in the longitudinal direction.
 3. A plate according toclaim 1 or 2, wherein said cut surface is formed at a portion of saidincident surface in a latitudinal direction.
 4. A plate according toclaim 1, wherein said cut surface projects from another plane of saidincident surface.
 5. A light guide plate according to claim 2, whereinsaid cut surface projects from another plane corresponding to the otherportion of said incident surface.
 6. A light guide plate according toclaim 3, wherein said cut surface projects from another planecorresponding to the other portion of said incident surface.
 7. A lightguide plate according to claim 4 or 5, wherein said cut surface projectsso that said cut surface is parallel to another plane corresponding tothe other portion of said incident surface, the projecting distance ofsaid projection being not greater than about 1 mm, and said cut surfacehaving a surface roughness of not greater than about 50 μm in terms of10-point average roughness (Rz).
 8. A light guide plate according toclaim 6, wherein said cut surface projects so that said cut surface isparallel to another plane corresponding to the other portion of saidincident surface, the projecting distance of said projection being notgreater than about 1 mm, and said cut surface having a surface roughnessof not greater than about 50 μm in terms of 10-point average roughness(Rz).
 9. A light guide plate according to claims 1, 2, 4 or 5, whereinsaid light guide is provided with a protruding portion which has alocation distant from said incident surface and functions forpositioning said light guide plate when being assembled.
 10. A lightguide plate according to claim 3, wherein said light guide is providedwith a protruding portion which has a location distant from saidincident surface and functions for positioning said light guide platewhen being assembled.
 11. A light guide plate according to claim 6,wherein said light guide is provided with a protruding portion which hasa location distant from said incident surface and functions forpositioning said light guide plate when being assembled.
 12. A method ofmanufacturing a light guide plate for a surface light source devicewhich comprises an incident surface in a longitudinal direction of aslender light source and an emission surface for emitting incident lightreceived at said incident surface in a direction perpendicular to thelongitudinal direction of said incidence surface, said light guide platebeing large in thickness at a position near said incident surface anddecreasing in thickness with increasing distance from the incidentsurface, the method comprising the steps of: (a) providing a mold with agate arranged at a position separated from a desired position of theincident surface, the gate being separated along a plane of the emissionsurface, the mold being further provided with an additional cavity toguide molten material from said gate to the desired position of theincident surface; (b) injecting molten material into the mold throughthe gate to form a guide plate portion and a protruding portion, theguide plate portion connecting to the protruding portion in a vicinityof the desired position of the incident surface; and (c) cutting theguide plate portion from the protruding portion in the vicinity of thedesired position of the incident surface, the cutting operationproviding a cut surface on the guide plate portion, the cut surfaceserving as at least a portion of the light incident surface.
 13. Amethod of manufacturing a light guide plate according to claim 12,wherein said gate faces an extension plane of the emission surface or aplane parallel with the emission surface.
 14. A light guide preformcomprising: a guide plate portion and a protruding portion, the guideplate portion connecting to the protruding portion in a vicinity of adesired position of a light incident surface, the guide plate portionhaving a thickness direction and a light emission surface, the lightemission surface being arranged perpendicular to the thicknessdirection, the guide plate portion having a thickness which decreaseswith increasing distance from the desired position of the incidentsurface, and the protruding portion being separated from the desiredposition of the incident surface, in a direction extending away from theguide plate portion along a plane encompassing the light emissionsurface, the protruding portion have a thickness sufficient to allowuniform flow of a molten injection molding material therethrough.
 15. Asurface light source device comprising: a light source; and a lightguide plate comprising: a single incidence surface next to which saidlight source is disposed for supplying light to said incidence surface;a generally rectangular emission surface; and a reflection surface whichopposes said emission surface and is provided with an uneven surfaceconfiguration for uniformizing emission from said emission surface,wherein a thickness of said light guide plate decreases from a thickside of said light guide plate to a thin side of said light guide plateopposing said thick side, said single incidence surface being providedonly at said thick side, said incidence surface includes a cut portionfrom which a protruding portion is cut, the protruding portion havingconnected said incidence surface to a molding gate which is positionedfrom said incidence surface by a predetermined distance and is alignedlongitudinally with a center portion of said incidence face, and saidprotruding portion has a plane shape which gets wider as approachingsaid incidence surface from said molding gate, and is generallysymmetric with respect to a center line running from said molding gatetoward said center portion of said incidence surface, wherein the lightguide plate is formed by injection molding using the molding gate sothat weld lines and warping in the light guide plate are reduced.
 16. Asurface light source device according to claim 15, wherein the lightguide plate being formed by injection molding flows through saidsupplemental cavity having a longitudinal length less than alongitudinal length of the incident surface.
 17. A surface light sourcedevice according to claim 15, wherein the emission surface extends in aplane, and the injection molding gate faces the plane of the emissionsurface.